As a conventional hydraulic pilot valve unit of the kind specified above, there is known a valve unit as shown, for example, in FIG. 1.
Stating in brief, this valve unit has a valve body 1 having a plurality of axially extending valve holes 2 formed within the valve body 1 along the circumference of the latter, and a valve spool 3 slidably mounted in each of these valve holes 2. Each of the valve spools 3 has formed therein a radial hole 4 for pressurized fluid which opens in a small diameter portion 3a formed on the approximately intermediate axial portion thereof, and an axial hole 5 for pressurized fluid, one end of which communicates with the radial hole 4 and the other end of which opens in the base end surface 3b of the valve spool.
The valve body 1 has further formed axially in turn therein pilot pressurized fluid outlet ports 6, an inlet port 8, and a reservoir port 10, all of which are communicated with each other through the valve holes 2. The inlet port 8 communicates with a pilot pressurized fluid supply pump 7, and the reservoir port 10 communicates with a fluid reservoir 9.
Pistons 11 are axially slidably mounted in the upper part of the valve body 1 at positions opposite to the valve spools 3 which are inserted in the above-mentioned plurality of valve holes 2, respectively. The upper end of each of the pistons 11 is biased against and kept in contact with the lower end surface of a pusher member 20 projecting from the valve body 1 and which is connected to an operating lever 18, by the resiliency of a spring 16.
Each valve spool 3 is slidably inserted in each of the valve holes 2 in such a manner that the base end surface 3b thereof faces the corresponding outlet port 6, and the leading end portion 3c thereof is fitted in a blind hole 12 formed in the base end of each of the pistons 11. The arrangement is made such that when the operating lever 18 is tilted in the direction shown by arrow each of the valve spools 3 is allowed to slidably move down in the drawing through the respective piston 11 and spring 16 against the resilient force of a spring 14.
The downward sliding movement of the valve spools 3 allows the pilot fluid under pressure which is supplied by the pump 7 into the inlet port 8 to flow through the holes 4 and 5 formed in each of the valve spools 3 into the respective outlet port 6.
The discharge rate of the pilot fluid under pressure which is allowed to flow into the outlet ports 6 is usually controlled in such a way as to increase in proportion to the stroke of the operating lever as shown, for example, by solid line in FIG. 3. Whilst, in the above-mentioned prior art hydraulic pilot valve unit, when the operating lever 18 is tilted, the valve spools 3 are slidably moved down in the same manner through the springs 16 in accordance with the movement downward sliding movement in the drawing) of the pistons 11. Therefore, when the operating lever 18 is tilted suddenly, the pistons 11 are slidably moved down quickly so as to deflect the springs 16 quickly. However, because of the weak resilient force of the springs 16, the deflection thereof will not provide momentarily a force to depress the valve spools 3 so that there is a time delay until the downward sliding movement of the valve spools 3 occurs. As a result, the relationship between the stroke of the operating lever 18 and the discharge rate of the pilot fluid under pressure will become as shown by dotted line in FIG. 3.
In brief, changes in the operating speed of the operating lever 18 result in changes in the discharge rate of pilot fluid under pressure relative to the stroke of the operating lever. When operating an implement of a hydraulic actuator or the like adapted to be operationally controlled by a pilot fluid actuated type change-over valve, etc. incorporating such a hydraulic pilot valve unit, it becomes difficult for the operator to manipulate the work implement of the construction vehicle to his will.